Bioastronomy 2002: Planets a'Plenty

Jul. 12, 2002

by Seth Shostak - Senior Astronomer

Seth Shostak, astronomer at the SETI Institute, is attending a week-long conference on life in the universe being held in Australia.

In 1995, the astronomy world went tilt as the first planet was detected around another, Sun-like star, 51 Pegasi. It was the biggest celestial discovery in years. These days, however, new planets have been turning up at a rate of more than one a month, and the announcement of yet another planetary world usually doesnt have an incendiary effect on the media.

Still, the audience at the Bioastronomy 2002 conference is keenly interested in such discoveries, and when Australian astronomer Chris Tinney offered up a world with only 20% the mass of Jupiter (the second smallest yet found around a normal star), he boosted the tally of extrasolar planets to 100. Thats a big enough sample that some of the astronomers involved in the search are stepping back, looking at the collection, and attempting to figure out what they can deduce about solar systems in general.

What they note is that 1% of solar-type stars have star-hugging giant planets (so-called "hot Jupiters"), similar to the world found around 51 Pegasi. Something more than 10% of solar-type stars have giant planets orbiting within a radial distance comparable to Jupiters distance from the Sun. Most of these are in highly egg-shaped (eccentric) orbits, which means that they probably undergo severe extremes of temperature as they move alternately closer to and farther from their suns.

In other words, all the planets found to date around solar-type stars have been massive (50 times Earths mass or more), and the majority of these bulky beasts are either in tight, inner orbits, or comet-like elliptical ones. Such planetary systems are very unlike our own, which might lead you to suspect that we Earthlings are lucky, and possibly alone.

But for those who prefer to think that we have cosmic company, theres no need for panic yet. Since 1995, the sensitivity of the Doppler wobble technique used by the planet hunters has improved, and the length of time theyve been busy searching has obviously increased. The consequence is that they are beginning to turn up planets with orbital shapes and dimensions similar to those found in our own solar system. Recently, California astronomer Geoff Marcy, who has more notches on his telescope than any other planet seeker, reported new members of the system around the star 55 Cancri. There are at least three big worlds paying homage to this star: one in a tight, 15-day orbit, one a bit farther out, orbiting in 44 days, and a hefty planet whose nearly circular orbit is slightly larger than Jupiters. This is a solar system that could, in principle, host an Earth-size world or two.

But do such smaller, more attractive planets actually exist? The extrasolar worlds discovered so far are all super-size (after all, thats what the Doppler wobble technique is best at finding). However, as Marcy, Tinney, and others point out, if you sort out these giant planets into bins, you find that the somewhat smaller ones greatly outnumber the bigger guys. Its just like animals: there are a lot more small ones than big ones. Now of course its a bit of a gamble to extrapolate this trend, but it certainly suggests that if we could only detect the really small, Earth-size worlds, we would find them in abundance. So what about that? When are the planet hunters going to bag some terrestrial-size prey? In the next decade, researchers figure to find all the detectible planets around the nearest one or two thousand solar-type stars, at least those as close, or closer, to their suns as Saturn is to ours. But all they can hope to find are big worlds. The Doppler wobble technique runs out of steam once you get down to about 20 Earth-mass planets or smaller. The natural churning of a stars hot outer surface sets this limit, not our instruments. No worries, as the Australians say. The search will go on using other approaches, virtually all of which involve putting telescopes in space. In 2007, the Kepler mission will lift off the pad, to be followed by the Space Interferometry Mission (SIM), Darwin, and ultimately the Terrestrial Planet Finder (TPF). These hi-tech eyes-in-the-sky will pick up where the ground-based Doppler wobble technique has left off, and tell us what, if anything, the roughly 90% of normal stars that dont seem to be hosting giants have to offer. One hopes, and not entirely without reason, that small worlds will make their inviting presence known.